Hello, arodlinux.
Thank you for using Apple Support Communities.
I understand that you're unable to create an email alias on iCloud. I'd like to help. What happens when you try to create one using these steps? Use email aliases on iCloud.com
Any error messages?
Make sure that the computer you're using is up to date and is compatible with iCloud: System requirements for iCloud
"Mac
- macOS Catalina
- Safari 9.1 or later, Firefox 45 or later, Google Chrome 54 or later, or Opera
PC
- Microsoft Windows 10*
- iCloud for Windows 10
- iTunes 12.7
- Firefox 45 or later, Google Chrome 54 or later (desktop mode only), Microsoft Edge, or Opera
*Windows 10 May 2019 Update or later is recommended for iCloud for Windows 10".
If possible try either another Wi-Fi connection or a different compatible web browser to see if that helps.
Keep me updated.
Cheers!